1. Field of the Invention
The present invention relates to a jig made from a material having high purity, superior resistance to high temperature heating cycles, and superb high thermal shock resistance, and thus is suitable for use as a jig for heat treatment used in the manufacture of semiconductors.
2. Description of the Background Art
A jig used for heat treatment of liner tubes, process tubes, wafer boats, and the like which are used in diffusion furnace in the manufacture of semiconductors must not only have a superior resistance to high temperature heating cycles (such as rapid heating and quenching), excellent high thermal shock resistance, and superb mechanical strength, it also must exhibit sufficient resistance to contamination to protect high-purity silicon wafers from being contaminated.
High-purity quartz has conventionally been used as a material in the jig used for such a heat treatment. The resistance of the high-purity quartz to a high temperature treatment, however, has become insufficient with the increases in the temperatures used for such a heat treatment. A silicon carbide material with silicon impregnated therein has been developed as a material to substitute for the high-purity quartz (U.S. Pat. No. 3,951,587). Impurities contained in this silicon carbide material, however, diffuse and come out on the surface, making the impurity level of this material higher than that of the high-purity quartz.
A silicon carbide sintered material prepared by providing a film of a ultrahigh purity silicon carbide by a CVD method (chemical vapor deposition method) on a silicon carbide matrix in which silicon has been impregnated has been proposed for use as a part in the manufacture of semiconductors to overcome this problem (Japanese patent No. 1682754). As disclosed in U.S. Pat. No. 3,157,541, formation of SiC films by the CVD method is carried out by either the pyrolysis of an organic silicon-containing compound, such as methyltrichlorosilane (CH.sub.3 SiCl.sub.3) or methyldichlorosilane (CH.sub.3 SiHCl.sub.2), or by the reaction of a silicon compound such as SiCl.sub.4 and a carbon compound such as CH.sub.4 to cause SiC to deposit with heating.
The formation of a SiC film is very effective for preventing the vaporization of impurities. However, when the SiC film is formed by the CVD method on the silicon carbide substrate in which silicon has been impregnated, resistance to high temperature heating cycles and high thermal shock resistance delicately change according to the conditions of the interface between the matrix and the SiC coating layer. This can cause cracks in or peeling of the SiC film, when the product is used as a semiconductor part which may suffer from a severe heat history.
In addition, part of the silicon impregnated in the silicon carbide matrix may easily vaporize and produce pores close to the surface of the substrate, resulting in impaired adhesion of the SiC film to the silicon carbide substrate. This will cause peeling of the SiC film from heat shock or high temperature heat cycles. Furthermore, if pinholes are produced due to impaired adhesion of the SiC film, part of the impregnated silicon may erode and dissolve out, thereby producing pores. Such pores permit impurities to accumulate and can be a source of such impurities.
A method for improving adhesion of the SiC film by causing SiC crystals to grow in the same orientation has been proposed (Japanese Patent Application Laid-open No. 210276/1988). Because the SiC film is formed at a temperature higher than the melting point of silicon in this method, a great number of pores are produced in the silicon carbide substrate, resulting in a decrease in the strength of the substrate.
Some inventors belonging to the inventive entity of the present invention have previously proposed a process for forming a SiC film on silicon-impregnated silicon carbide ceramics without producing pores. The process comprises heating a silicon carbide ceramic material in which silicon has been impregnated at a temperature of 850.degree. C. to 1000.degree. C. under a reduced pressure, introducing a reaction gas for forming SiC, thereby producing a SiC film while raising the temperature from 1200.degree. C. to 1400.degree. C. without cooling the product (Japanese Patent Application Laid-open No. 65374/1992). In this process, formation of the SiC film is initiated at a temperature below 1000.degree. C. at which the rate of vaporization of silicon is small, then the temperature is raised to 1000.degree. C. or higher. The film which has been formed at lower temperatures controls the vaporization of the silicon, even though the temperature is raised to 1000.degree. C. or higher, whereby the number of pores which are formed can be reduced.
To resolve the above-mentioned problems existing in the conventional process for producing an SiC film by chemical vapor deposition on the silicon-impregnated silicon carbide substrate used as semiconductor manufacturing parts, the present inventors have conducted extensive studies on the relationship between the properties at the interface of the substrate and the SiC coating layer and heat resistance characteristics of the resulting silicon carbide substrate, the factors for the formation of pores in the course of the SiC film production, and the like, on the basis of the above-mentioned process for forming a SiC film wherein the film formation is initiated in a temperature range of 850.degree. C. to 1000.degree. C.